Shelbi L Russell Project Summary Bacterial symbionts are ubiquitous among eukaryotes and are responsible for some of the most radical lifestyles in the natural world. For example, microbial symbiosis enables hydrothermal vent ecosystems to subsist on inorganic energy and carbon sources and plant-feeding insect communities to thrive on nitrogen-deficient diets. Often living with one partner inside the other, these associations require complex cellular mechanisms to ensure that conflict does not arise between host and symbiont. Reliable transmission mechanisms to reach new hosts are vital to stabilizing associations over evolutionary time. However, very little is known about the molecular mechanisms underlying these processes because the majority of endosymbionts are unculturable, and often the hosts are as well. Here, I propose to use ?Drosophila? fruit flies and their ?Wolbachia? endosymbionts as models for understanding host-symbiont interactions and the molecular mechanisms mediating symbiont transmission. ?Wolbachia ?is one of the most abundant intracellular symbionts in nature by virtue of its ability to associate with the host germline and manipulate host reproduction for vertical transmission. It is also occasionally beneficial to its hosts by promoting pathogen resistance and performing necessary cellular tasks. These traits make this bacterium useful for applications in disease vector control. While Wolbachia i? s faithfully inherited through the germline in all associations examined to date, horizontal transmission between contemporary hosts, of the same and different species, is common throughout their evolutionary history and can be recapitulated in the lab. During the K99 funding period, I will use the ?D. melanogaster?-?Wolbachia ?system to characterize and identify the genes/pathways necessary for endosymbiont transmission within and between cells. This will be accomplished in two aims: In Aim 1, I will use ?Wolbachia?-infected ?Drosophila c? ell lines to explore the functional mechanisms and evolutionary outcomes of mixed strain infections. In Aim 2, I will characterize the symbiont and host linker proteins ?Wolbachia ?uses for KHC-dependent microtubule-based motility. I will use the results of this work during the R00 phase to explore how intracellular and cell-to-cell transfer mechanisms integrate in the whole fly for vertical transmission through the germline and horizontal transmission between host individuals. Thus, this work will provide mechanistic insight into the transmission strategies employed by endosymbionts around the world.